The present invention relates to a scanning optical system which converges a plurality of beams (emitted from a plurality of light emitting points) by a line image formation lens to focus in an auxiliary scanning direction in the vicinity of a reflecting surface of a deflecting system, dynamically deflects the beams by the deflecting system in a main scanning direction, and converges the dynamically deflected beams by an imaging optical system into point-like beam spots on a scan target surface.
Scanning optical systems are widely used for electrophotographic laser beam printers, digital photocopiers, laser fax machines, laser plotters, etc. for scanning the surface of a photosensitive body (photosensitive drum, etc.) as the scan target surface by a modulated beam.
One of conventional scanning optical systems is configured as follows. That is, the scanning optical system converges a laser beam (which has been ON-OFF modulated according to image information) by a line image formation lens to focus in the auxiliary scanning direction in the vicinity of a reflecting surface of the deflecting system, dynamically deflects the laser beam by the deflecting system in the main scanning direction, and converges the dynamically deflected laser beam by the imaging optical system into a spot beam on the scan target surface. By the above mentioned mechanism, the scanning optical system scans the ON-OFF modulated spot beam on the scan target surface in the main scanning direction at a constant speed, and thereby forms a two-dimensional image composed of a plurality of dots on the scan target surface.
Meanwhile, when a light source that emits a plurality of laser beams from a plurality of light emitting points (e.g. laser diodes) is used, a plurality of line images arranged in the auxiliary scanning direction can be formed in the vicinity of the reflecting surface of the deflecting system by use of the line image formation lens. Therefore, through the deflecting system and the imaging optical system, a plurality of spot beams arranged in the auxiliary scanning direction are formed on the scan target surface. By such a scanning optical system (the so-called xe2x80x9cmultibeam optical systemxe2x80x9d), a plurality of scan lines can be simultaneously drawn on the scan target surface in one scan by one reflecting surface of the deflecting system. Therefore, with the ON-OFF modulation of each beam according to the image information, high speed printing can be realized.
In addition to the above multibeam optical system, printers capable of switching the recording density (in order to change the printing speed) depending on the purpose are well known. This type of printer switches the recording density in the main scanning direction by changing the modulating cycle, while switching the recording density in the auxiliary scanning direction by changing the rotational speed of the photosensitive drum.
However, in the aforementioned multibeam optical system employing a light source having a plurality of light emitting points, the interval between the simultaneously scanned scan lines (hereinafter, referred to as xe2x80x9cscan line intervalxe2x80x9d) is fixed. With the fixed scan line interval, when the recording density in the auxiliary scanning direction is switched by changing the rotational speed of the photosensitive drum as above, a mismatch occurs between the interval between the simultaneously scanned scan lines (fixed scan line interval) and the interval between successively scanned scan lines.
To avoid the problem, there has been proposed a method capable of altering the recording density by changing the rotational speed of the photosensitive drum while maintaining an even interval between the scan lines formed on the scan target surface by the multibeam scanning optical system (Japanese Patent Provisional Publication No. SHO57-54914, for example). The scanning optical system disclosed in this publication employs an afocal anamorphic zoom lens system as the line image formation lens and synchronously moves a plurality of lenses of the afocal anamorphic zoom lens system in the optical axis direction. With this structure, the magnification of the whole line image formation lens can be changed while maintaining xe2x80x9cfocus positionsxe2x80x9d of the beams (where the beams after passing through the line image formation lens focus in the auxiliary scanning direction in the vicinity of the reflecting surface of the deflecting system) at fixed positions, by which the scan line interval can be adjusted continuously according to the rotational speed of the photosensitive drum.
However, when an anamorphic movable lens (cylindrical lens, etc.) is installed in the line image formation lens, a relative tilt of a lens surface caused by the movement of the anamorphic movable lens introduces a twist in the shape of the wavefront. In order to continuously change the magnification of the line image formation lens while suppressing the error caused by the relative tilt, a movement control mechanism composed of high precision parts becomes necessary and it drives up the costs.
The present invention is advantageous in that it provides a scanning optical system capable of resolving the above problems and realizing the switching of the scan line interval in the auxiliary scanning direction in the multibeam optical system extremely easily, without the need of a movement control mechanism composed of high precision parts.
According to an aspect of the invention, there is provided a scanning optical system for dynamically deflecting a plurality of beams simultaneously and thereby scanning the beams in a main scanning direction on a scan target surface. The scanning optical system is provided with a light source having a plurality of light emitting points which emit the plurality of beams and a collimator lens which collimates the plurality of beams, the plurality of beams being emitted from the light source as a plurality of collimated beams collimated by the collimator lens, a first optical system including a first fixed lens group placed on a light source side of the first optical system and a movable lens group having negative finite transverse magnification with respect to images formed by the first fixed lens group, the first optical system converging each of the plurality of beams emitted from the light source in an auxiliary scanning direction perpendicular to the main scanning direction.
The scanning optical system is further provided with a moving mechanism which holds the movable lens group to be movable along an optical axis of the collimator lens and selectively stops the movable lens group at a first position and a second position only, the first and second positions being determined so that transverse magnification Mp1 of the movable lens group with respect to the images formed by the first fixed lens group when the movable lens group is placed at the first position and transverse magnification Mp2 of the movable lens group with respect to the images formed by the first fixed lens group when the movable lens group is placed at the second position will satisfy Mp1xc3x97Mp2=1 . . . (1). The scanning optical system further provided with a deflecting system that dynamically deflects the plurality of beams simultaneously in the main scanning direction at a position in the vicinity of a line image formation position where a plurality of line images are formed by the convergence of the beams in the auxiliary scanning direction by the first optical system, and a second optical system which converges the dynamically deflected beams in the main scanning direction and in the auxiliary scanning direction to focus in the vicinity of the scan target surface and thereby forms a plurality of scan lines on the scan target surface.
With this configuration, the moving mechanism selectively stops the movable lens group only at the first and second positions which satisfy the condition (1), by which the distance between the object point and image point of the movable lens group when the movable lens is placed at the first position becomes equal to that when the movable lens is placed at the second position regardless of actual figures of the transverse magnifications Mp1 and Mp2 at the two positions. Consequently, the line images are formed at fixed positions even though the focal length (and thereby the magnification) of the whole first optical system can be changed between two values. Therefore, the switching of the scan line interval in the auxiliary scanning direction can be realized extremely easily by use of a simple moving mechanism that enables the movement between the two positions, without the need of a movement control mechanism composed of high precision parts.
Optionally, an interval P1 between the scan lines when the movable lens group is placed at the first position and an interval P2 between the scan lines when the movable lens group is placed at the second position may satisfy:
Mp1=xe2x88x92(P1/P2)1/2=1/Mp2xe2x80x83xe2x80x83(2).
Still optionally, the first optical system may consist of a first fixed lens group having negative refractive power in the auxiliary scanning direction and thereby forming the images as virtual images and a movable lens group having positive refractive power in the auxiliary scanning direction. Incidentally, in this case, in order to satisfy the condition xe2x80x9cnegative finite transverse magnificationxe2x80x9d of the movable lens group, the moving mechanism is required to move the movable lens group so that the front focal point of the movable lens group (either at the first position or at the second position) will be nearer to the deflecting system than the virtual images formed by the first fixed lens group.
In a particular case, the transverse magnification Mp1 of the movable lens group in the auxiliary scanning direction when the movable lens group is placed at the first position may be approximately xe2x88x921.41, and the transverse magnification Mp2 of the movable lens group in the auxiliary scanning direction when the movable lens group is placed at the second position may be approximately xe2x88x920.71.
Optionally, the first optical system may include the first fixed lens group having positive refractive power in the auxiliary scanning direction and thereby forming the images as real images, a movable lens group having negative refractive power in the auxiliary scanning direction and thereby forming virtual images of the real images, and a second fixed lens group having positive refractive power in the auxiliary scanning direction and thereby forming real images of the virtual images. In this case, in order to satisfy the condition xe2x80x9cnegative finite transverse magnificationxe2x80x9d of the movable lens group, the moving mechanism is required to move the movable lens group so that the rear focal point of the movable lens group (either at the first position or at the second position) will be nearer to the light source than the focal point of the first fixed lens group.
Still optionally, the transverse magnification Mp1 of the movable lens group in the auxiliary scanning direction when the movable lens group is placed at the first position may be approximately xe2x88x921.22, and the transverse magnification Mp2 of the movable lens group in the auxiliary scanning direction when the movable lens group is placed at the second position may be approximately xe2x88x920.82.
In a particular case, the moving mechanism includes a first fixed mount on which the first fixed lens group is mounted, a second fixed mount placed at a preset distance from the first fixed mount, a movable mount placed between the first and second fixed mounts on which the movable lens group is mounted, a guide held by the first and second fixed mounts and inserted into a through hole of the movable mount, a screw rotatably held by the first and second fixed mounts and inserted into a through hole of the movable mount having an engaging mechanism for smoothly engaging with the screw, and a rotating mechanism for rotating the screw. In this structure, the movable lens group is stopped at the first position when the movable mount makes contact with the first fixed mount, and the movable lens group is stopped at the second position when the movable mount makes contact with the second fixed mount.
Of course, the first optical system may also consist of a first fixed lens group having positive refractive power in the auxiliary scanning direction and thereby forming the images as real images and a movable lens group having positive refractive power in the auxiliary scanning direction. In this case, the moving mechanism moves the movable lens group so that the front focal point of the movable lens group (either at the first position or at the second position) will be nearer to the deflecting system than the real images formed by the first fixed lens group.